[Bug c/118827] New: riscv scalar and vector are all saved when __attribute__((interrupt)) used

[fanghuaqi at vip dot qq.com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=118827

            Bug ID: 118827
           Summary: riscv scalar and vector are all saved when
                    __attribute__((interrupt)) used
           Product: gcc
           Version: 15.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: fanghuaqi at vip dot qq.com
  Target Milestone: ---

Hello,

I met a strange issue when using __attribute__((interrupt)) to define a vector
interrupt function, and found that when I call a function from this interrupt
function, all scalar and vector registers are saved and restore when compiled
with -march=rv64gcv -mabi=lp64d -O0

Code as below:

#include <stdio.h>

static unsigned int cnt;

void delay(int ms)
{
    for (volatile int i =0; i < ms; i ++) {
        asm volatile ("nop");
    }
}

__attribute__((interrupt)) void test(void)
{
    cnt ++;
    delay(cnt);
}

void main(void)
{
    while (1) {
        printf("cnt is %d\n", cnt);
    }
}

godbolt link: https://godbolt.org/z/3hKaahYsP

>From gcc 14.x, the vector registers are also saved and restored if I call a
subfunction from interrupt function.

How can I make this interrupt attribute marked function save less registers on
stack, since sometimes the subfunction is really simple, but call it will cost
a lot stack save and restore.

Thanks for any input